1. Field of the Invention
The present invention relates to a recording method and a recording device for an optical information recording medium that optically records and reproduces data, and particularly to a method for generating a recording pulse waveform.
2. Description of the Prior Art
Recently, optical disks, optical cards and optical tapes have been proposed and developed as a medium for optically recording data. Among them the optical disk has received attention as a large-capacity medium that can record and reproduce much data at high density.
For example, a phase change type optical disk records and reproduces data by the method described below. A laser beam having a power higher than a reproducing power (this power level is called the recording power and is denoted by Pw) is focused by an optical head and applied to a recording film of the optical disk so that a temperature of the recording film is raised above its melting point. Then, the melted portion is cooled rapidly when the laser beam passes so that a mark is formed having an amorphous state. In addition, when a laser beam having a power such that it can raise the temperature of the recording film above a crystallizing temperature and below the melting point (this power level is called an erasing power and is denoted by Pe) is focused and applied, the applied portion of the recording film becomes crystalline.
Thus, a recording pattern including marks as amorphous areas and spaces as crystalline areas is formed corresponding to a data signal. Then, the data is reproduced by utilizing a difference of reflection factor between the crystalline area and the amorphous area.
As described above, in order to form marks on a medium, it is required to modulate the power level of the laser beam at least between the erasing power and the recording power for light emission. A pulse waveform that is used for this modulation operation is called a recording pulse. Various recording methods of forming one mark by using a plurality of recording pulses have already been disclosed. The plurality of recording pulses is called a recording pulse train. In addition, a recording method for modulating the power level of the laser beam between the recording power, the erasing power and a power lower than the erasing power (this power level is called a bottom power) , which causes light to be emitted, is also disclosed. Furthermore, a recording method is also disclosed in which a recording pulse having a power lower than the erasing power (this pulse may also be called a cooling pulse) is added to the end of the recording pulse train.
In addition, a mark edge recording technique, that is a typical technique for DVDs and the like, utilizes a recording method in which a recording pulse is split into the above-mentioned recording pulse train when recording a long mark. A width of a first pulse (this is called a front end pulse) is set to a larger value than a width of a middle pulse or a last pulse (this is called a rear end pulse). Considering the effect of excessive heat that is conducted from a front portion of a mark, this reduces distortion of a shape of the recording mark for more accurate recording by decreasing the amount of heat that is applied to the recording film when recording the end portion of a mark lower than when recording a front portion of a mark.
In the case of the mark edge recording method, a difference in thermal characteristics of the optical disk affects the state of a recording mark and the level of thermal interference between recording marks. Namely, shapes of recording marks are different between disks even if the same recording pulse waveform is used for recording. As a result, a recording mark edge can be shifted from an ideal position depending on the disk, resulting in deterioration of quality of a reproduced signal.
Therefore, a method has been proposed in which a front end pulse edge position or a rear end pulse edge position is corrected to be at an optimal position for each disk so that recording marks can be recorded at ideal edge positions for each disk.
For example, there has been a method disclosed in which the front end pulse edge position or the rear end pulse edge position is corrected in accordance with a corrected value. The corrected value represents a combination of a code length corresponding to the mark to be recorded (this is called a record code length) and code lengths corresponding to the spaces before and after the mark (these are called a previous code length and a next code length, respectively) (for example, see Japanese unexamined patent publication No. 7-129959, pages 4-5 and FIG. 2).
In addition, there is also a test recording method disclosed for correcting the front end pulse edge position or the rear end pulse edge position. In this method, prior to recording a real information signal, a data pattern having a specific period (this is called a test pattern) is recorded. Then, the recorded test signal is reproduced so that the reproduced signal can be measured to determine a shift of a mark edge.
However, the above-mentioned conventional recording method has a problem that depending on a waveform of the recording pulse or thermal characteristics of the medium, variation in the actual mark edge becomes excessively sensitive or insensitive to the amount by which the edge position of the recording pulse is corrected. Hereinafter, the problem will be described.
For example, when recording on a medium having a structure that diffuses only little heat upon recording (this may also be called a slow cooling structure) by widening the recording pulse width, there is a tendency for the amount of variation of the mark edge to be smaller with respect to the amount of variation of the edge position of the recording pulse. This occurs because the effect of heat accumulated in the recording film of the medium is larger than the effect of the thermal variation due to the variation of the pulse edge.
As a result, even if the front end edge of the recording pulse is changed in a stepping manner by a resolution rp1 like a, b, c, d and e as shown in FIG. 13(a) for example, the actual mark edge variation becomes smaller than that, and a resolution of the edge position becomes rm2 as shown in FIG. 13(b). When this mark is reproduced, because the leading edge position of the reproduced signal corresponds to the mark edge position, the resolution rs2 of the leading edge position of the reproduced signal becomes smaller than rp1 as shown in FIG. 13(c).
Thus, if a medium is used that has the actual mark edge variation smaller than the edge position variation of the recording pulse, it is necessary to increase the number of setting steps of the edge position of the recording pulse so that the range in which the recording pulse edge position is set can be enlarged for forming marks having the desired mark edge positions. For this reason, it is required to use a delay line having higher numbers of setting steps, which leads to a complicated circuit and an increase of manufacturing cost.
On the other hand, when a width of the recording pulse for forming the mark edge is small, there is a tendency that the mark edge variation becomes larger than the edge position variation of the recording pulse, for example. This is because the recording pulse width varies relatively largely in accordance with the edge position variation of the recording pulse, so that the variation of the total energy that is used for forming mark edges is too large to be ignored.
As a result, if the rear end edge of the recording pulse is changed in a stepping manner by a resolution rp1 like a, b, c, d and e as shown in FIG. 14(a) for example the actual mark edge variation becomes larger than that, and a resolution of the edge position becomes rm2 as shown in FIG. 14(b). When this mark is reproduced, the resolution rs2 of the trailing edge position variation of the reproduced signal becomes larger than rp1 as shown in FIG. 14(c).
Thus, if a medium is used that has the actual mark edge variation larger than the edge position variation of the recording pulse, it is necessary to decrease a resolution of correction of the recording pulse edge position for forming marks having the desired mark edge positions. However, in order to decrease the resolution of correction, it is necessary to use a delay line having high accuracy in a circuit for correcting the record signal edge, which also leads to a complicated circuit and an increase of manufacturing cost.
In addition, if the correction amount of the edge position of the recording pulse is different from the actual mark edge variation, there may be many occasions where an expected correction amount of the mark edge cannot be obtained, even if the edge position of the recording pulse is corrected by the test recording. Therefore, the test recording may be repeated until the mark edge converges at a predetermined position. Thus, there is a problem that it takes a long time for the recording device to become really able to record data.
Furthermore, if a recording line speed of a medium and a channel clock frequency are increased so as to increase a transfer rate for recording, it becomes necessary to decrease the resolution of the correction of the edge position of the recording pulse, which leads to a complicated circuit and an increase in manufacturing cost in the same manner as described above.